How to properly bake normal maps?
As staded by @Yvain baking one object maps to another involves projecting them on the baking faces. If you move the object away, or the faces are not pointing each other, you'll not get expected results.

 

While it would be cool not to have the object in the same place for baking highres to low, a similar behaviour would limit the user possibilities.

This is just the tip of the iceberg, as baking normal maps involves many aspects, such as resolution, uv maps, shading, cages...

I would suggest to have a look of these tutorial:


The Fundamentals of Perfect Baking (not blender related)
Blender 2.71 - Cycles Baking using Cages